A linear accelerator structure accelerates charged particles of a specific mass/charge ratio which are injected into the accelerator at a specific injection energy. Radio frequency (rf) linear accelerators have been known for many years from the field of nuclear physics where they have been employed to accelerate heavy ions. More recently, rf accelerators have been used in semiconductor wafer processing. Typically, a beam of ions of a required species (such as boron, phosphorous, arsenic or antimony) is produced and directed at a wafer so that the ions become implanted under the surface of that wafer. Although electrostatic acceleration systems are suitable for producing beams of singly charged ions of 200 keV or more, it has been recognised that the desirable characteristics (for certain applications) of relatively high beam current and relatively high beam energy can be achieved by including an rf accelerator in the ion implanter device.
The use of rf linear accelerators for implantation of ions into semiconductor wafers has been suggested at least since 1976 in "Upgrading of Single Stage Accelerators" by K. Bethge et al, pages 461-468, Proceedings of the Fourth Conference on the Scientific & Industrial Applications of Small Accelerators, North Texas State University, Oct. 27-29, 1976; and in "Heavy Ion Post-acceleration on the Heidelberg MP Tandem Accelerator", edited by J. P. Wurm, Max Planck Institute for Nuclear Physics, Heidelberg, May 1976. U.S. Pat. No. 4,667,111 discloses an ion implanter incorporating a radio frequency linear accelerator to provide ultimate beam energies as high as 2 MeV or more.
As discussed by Glavish et al in "Production of high energy ion implanters using radio frequency acceleration", Nuclear Instruments and Methods in Physics Research B21 (1987), at pages 264 to 269, it is necessary that each resonator in the rf accelerator be kept in precise tune and matched to its amplifier, for example by feedback control of a movable plate capacitor. The resonators tend to be sensitive to thermal and mechanical disturbances as they are part of highly tuned systems, with Q values between 1000 and 2000. It is also important that the amplitude and phase of the rf voltage at the acceleration electrode be controlled. In one arrangement, a signal from the inductive or capacitative probes associated with each cavity is compared with the desired phase and amplitude derived from a master oscillator via a precision phase shifter. Using a microprocessor, a "parameter set" for a given ion beam energy and species may be developed. Phase may be held to about 1.degree. and amplitude to within 1%.
U.S. Pat. No. 5,801,488 also describes the control of an rf accelerating device. Here, a control unit determines the respective value is of phase and rf power, based upon a predetermined programmed algorithm, to obtain a target energy which is set by an operator. The controller adjusts the phase and amplitude under feedback control. In "The Development of a Beam Line using an RFQ and 3-Gap RF Accelerators for High Energy Ion Implanters", presented by Fujisawa et al at IIT in Kyoto, Japan, Jun. 24, 1998, a personal computer is employed to control phase and amplitude to an RFQ and 3-gap rf beam line. Again, phase is controlled to around 1.degree. and amplitude to around 0.5%.
It will thus be appreciated by those skilled in the art that the precision and stability of the system relies upon the ability to generate a signal, for each resonator, which has a precise phase and amplitude. It is also important that the relative phase shift between resonators is accurately maintained.